What exactly is the Dew Point Temperature?

The conditions were just right yesterday to delve a little deeper into what exactly is the "dew point".

The Weather Bureau observations show quite a few different types of readings:

The temperature (Temp) is the actual air temperature outside, measured out of the direct sunlight. This is the one we most often hear about.

The apparent temperature (App Temp) is what it feels like, when you take the wind and moisture in the air into consideration.

The dew point temperature (Dew Point) is what I'm focussing on today. It gives a good indication of how much moisture is in the air.

So does relative humidity - so why don't we just use that? I prefer the dew point, because the relative humidity (Rel Hum) is exactly that, its relative. Relative to what the air temperature is doing, which can not show the true moisture in the air.

We started Wednesday evening hot and dry, with a low dew point in a dry northerly wind. Then the cool change arrived and the temperature dropped, but the dew point suddenly rose as the hot, dry air was replaced by cool, moist air. It really felt different outside - and many people kept their air conditioning on because yes, it was cooler, but it wasn't overly "refreshing".

The dew point remained up on 15C through the rest of the night, and the air temperature can't fall below the dew point, so we didn't go any lower than 15.9C.

Farmers have an old trick where they look at what the dew point is at 3pm in the afternoon, to determine if there will be a frost. If the airmass doesn't change, then that's roughly how low the temperature will be overnight. It doesn't work if a cold front moves through though (so I'd alway recommend checking my forecast :))!


The dew point is why 25C can feel so different from Melbourne to Brisbane. Melbourne is often between 5 and 10, while Sydney are over 15C and Brisbane are over 20C. That could mean that 25C actually feels like 19C in Melbourne, 24C in Sydney and 28C in Brisbane! 

I hope this helps shed some light on the intriguing "Dew Point". I'll be referencing it a lot more in the current conditions, taking you along on my #janesweather journey. See you tonight!

Jane Bunn